The game does not allow to claim a ferry route with locomotives only, though it is not against the rules. I met this bug both on the Europe map and on the Asia map on the iPad version of TtR. It happens very often, not always though.
Screenshot included.
If I want to go from Bombay to Colombo for example, and it so happens that I have neither black nor red cards but four locomotives instead, I won't be able to do it.

The game does not allow to claim a ferry route with locomotives only, though it is not against the rules. I met this bug both on the Europe map and on the Asia map on the iPad version of TtR. It happens very often, not always though.
Screenshot included.
If I want to go from Bombay to Colombo for example, and it so happens that I have neither black nor red cards but four locomotives instead, I won't be able to do it.

I can confirm that if one tries to use 4 locos on Bombay to Columbo it will not allow you to claim, even if that is your first route to claim so it definite is a bug.

Likewise on the Asia map it seems any color route with a loco won't allow you to claim with only locos. Nor it would seem any of route with one loco and the rest grey does it allow me to claim those routes with only locomotives.

I am sure it is not a bug. I think that the developers just did not think it thoroughly to the end. What I mean is when you try to claim a grey ferry route the game says that it can't figure out what color I mean to use with the loco card, and there is some logic in it. On the Europe map there are only grey ferry routes and if you draw only locos this can prevent you from a mistake to spend locos instead of cards of some color which are less precious anyway. The wrong thing is that the same mechanism is applied on the Asia map where there are ferry routes of certain colors not only grey ferry routes. Anyhow I think that the mechanism is a mistake in its origin. The message that the game can't figure out what color I mean to use with the loco card is only justified when I do not have enough locos to claim the ferry route only with locos. I think that players should be given the same freedom as in the board game. If I try to claim a route with a loco card and I have enough locos why prevent me from it? It even can cost somebody a victory in case with the colored ferry routes!!! So that is the issue! And I hope that somebody from DoW TtR team will see this post and correct the mechanism.

So happy that this issue, I reported 3 years ago, is finally fixed. Will be holding my fingers crossed, that some day (maybe a few years from now) digital Barbarians of Small World will play as they should.